Lead-acid electrochemical cells have been commercially successful as power cells for over one hundred years. For example, lead-acid batteries are widely used for starting, lighting, and ignition (“SLI”) applications in the automotive industry.
As an alternative to lead-acid batteries, nickel-metal hydride (“Ni-MH”) and lithium-ion (“Li-ion”) batteries have been used for electric and hybrid-electric vehicle applications. Despite their higher cost, Ni-MH and Li-ion electro-chemistries have been favored over lead-acid electrochemistry for some applications due to their higher specific energy and energy density compared to prior known lead-acid batteries.
Some lead-acid electrochemical batteries are made up of multiple electrochemical cells. Each cell includes a positive plate, a negative plate, a separator, electrolyte, and current collector. In some batteries, a casing surrounds stacked electrodes to form a module; in others, each cell is isolated in its own casing. For example, a 12-volt car battery has 6 cells of 2-volts each, each in a separate section of the battery casing to isolate each cell.
The positive and negative plates may have a substrate and active material applied over the substrate. In conventional lead-acid batteries, the substrate is typically a lead-alloy plate. The lead-alloy may be one of lead-alloy; lead-tin alloy; lead-tin-calcium alloy; or other suitable alloys, The plate is typically used to retain the active material and collect and distribute current throughout the plate.
Substrates may also be made of wires. For example, wires may be woven into a sheet suitable for performing the same functions as an expanded grid.
Prior known substrates are subject to corrosion. Corrosion may be driven by either the electrochemistry of the electrolyte/electrode couple or the electrical potential region at which the electrodes and active materials are operated. This phenomenon occurs in various electrochemistries and in particular in the lead-acid case, where the active material takes part in oxidation and reduction reactions. Specifically, the sulfuric acid electrolyte may change concentration during charge and discharge of the battery, and may attack the substrate material. Over the life of the battery, corrosion consumes the substrate material. Corrosion due to repeated cycling may cause the substrate to lose its function of supporting the active material; or collecting and distributing current.
Some corrosion may be desirable. Upon activation, a limited amount of corrosion may help bond the active material to the substrate, both chemically and electrically. Preferably the corrosion process would be stopped after this beneficial effect has been obtained. Arresting corrosion, however, may be difficult or even impossible. The core mechanism of the electrochemical cell may rely on the corrosion, or reduction-oxidation, reaction. Upon cycling, the active materials may change over time. In particular, it may change in volume and dilate. The oxide layer at the grid/active material interface may break down and expose new lead to the corrosion reaction. This process repeats with continued cycling, deteriorating the substrate. In practice, it may not be possible to stop the corrosion reaction at the optimal time.
To compensate for the corrosion, battery designers may be forced to add additional lead material. This addition increases the weight of the battery, thus reducing specific power and energy. Alternatively, designers may select alloying elements to inhibit corrosion. Designers may also accept that the grid has a hunted life and warrant their products only up to the limits of cycle life imposed by corrosion processes.
Thus there remains a need for substrate materials that resist corrosion after the beneficial adhesion layer between active material and substrate is formed. Preferably the substrate would corrode slowly, if at all. This may enhance cycle life of the energy storage or conversion device in which the material is used.
These above corrosion effects may also occur in alternative electro-chemistries, such as those of Ni-MH and Li-ion batteries. Therefore, these alternative batteries may also benefit from improved alloy materials that better resist corrosion.